The soundtrack of the HL2 Mod Dystopia is also great and very atmospheric, although it goes in a rather different, heavier direction. Examples: spider trap, Conflict AriaDystopia is also free on Steam, and the full OST comes with the game.

On https://soundcloud.com/nervous_testpilot, nervous_testpilot has recently been putting new work for Frozen Endzone up for streaming. It is much like the Frozen Synapse soundtrack, but features trance music as well. Give it a listen, those new tracks rock!

Sorry for necroing an old thread, but this is a top Google result for "music like frozen synapse" and related searches. I, for myself, would have been helped greatly if Paul's soundcloud were mentioned here, as I spent an hour Googling and ended up finding it through some obscure social media post ^^.